Support PHP 8.4.1

Since PHP 8.4.1 there are lots of deprecated warnings. Whilst they are only warnings we should look to fix them. ``` PHP Deprecated: can_impersonate(): Implicitly marking parameter $guard as nullable is deprecated, the explicit nullable type must be used instead in /Users/lloyd/Documents/work/clear/*/vendor/lab404/laravel-impersonate/src/helpers.php on line 13 Deprecated: can_impersonate(): Implicitly marking parameter $guard as nullable is deprecated, the explicit nullable type must be used instead in /Users/lloyd/Documents/work/clear/*/vendor/lab404/laravel-impersonate/src/helpers.php on line 13 PHP Deprecated: can_be_impersonated(): Implicitly marking parameter $guard as nullable is deprecated, the explicit nullable type must be used instead in /Users/lloyd/Documents/work/clear/*/vendor/lab404/laravel-impersonate/src/helpers.php on line 31 Deprecated: can_be_impersonated(): Implicitly marking parameter $guard as nullable is deprecated, the explicit nullable type must be used instead in /Users/lloyd/Documents/work/clear/*/vendor/lab404/laravel-impersonate/src/helpers.php on line 31 PHP Deprecated: is_impersonating(): Implicitly marking parameter $guard as nullable is deprecated, the explicit nullable type must be used instead in /Users/lloyd/Documents/work/clear/*/vendor/lab404/laravel-impersonate/src/helpers.php on line 48 Deprecated: is_impersonating(): Implicitly marking parameter $guard as nullable is deprecated, the explicit nullable type must be used instead in /Users/lloyd/Documents/work/clear/***/vendor/lab404/laravel-impersonate/src/helpers.php on line 48 ``` I believe this is a fairly easy fix.